Method for the production of borosilicate glass with a surface suitable for modification glass obtained according to said method and the use thereof

ABSTRACT

The method of making borosilicate glass with a surface having reactive SiOH groups on it includes preparing a borosilicate glass melt and dissolving at least 30 mMol per liter of water in the borosilicate glass melt. The borosilicate glass contains from 70 to 87 percent by weight, SiO 2 ; from 7 to 15 percent by weight, B 2 O 3 ; from 0 to 8 percent by weight, Al 2 O 3 ; from 0 to 8 percent by weight, Na 2 O and from 0 to 8 percent by weight of K 2 O. The borosilicate glass with the easily modified reactive surface can be used as a substrate for chemically covalent immobilization of reactive substances. This substrate can be used to make a biochemical chip, such as a DNA or gene chip, or dirt-proof window glass.

BACKGROUND OF THE INVENTION

The present invention relates to the production of borosilicate glass,in particular a borosilicate glass substrate with a surface suitable formodification, and the glass obtained according to the method accordingto the invention, and the use thereof.

The use of glass as a carrier substrate for a large number of uses isknown per se. According to the usual procedure, a desired chemicalsubstrate (modifying agent), such as biomolecules, is immobilized on theglass surface. This is usually carried out using SiOH groups, which arefreely available on the glass surface. To obtain a sufficient number ordensity of modifying agents, the number of reactive SiOH groups must beincreased. This can be accomplished, e.g., using treatment in a gasplasma. A further method for increasing the surface reactivity of glassis to treat it with alkali hydroxides, in particular sodium hydroxide.Glass surfaces that have been treated in this manner then react easilywith other reagents, and a coated glass surface is obtained. In thismanner, it is possible to covalently bond the surfaces of glass with alarge number of compounds to obtain certain properties, such asdirt-proof properties, by bonding with silanes, or it is possible toobtain certain reactions, e.g., with biomolecules.

The term “biochip” refers to devices that have a biological or organicmaterial that is immobilized on the solid carrier substrate. Siliconwafers, thin glass plates, plastic or nylon membranes serve as a commoncarrier substrate for chips of this type. Aluminum has also been used ascarrier material. Glass is typically preferred, however, due to itssurface properties, low natural fluorescence compared to other plasticmaterials, and its resistance to chemical substances and temperaturestability. It is also resistant to aging. It has been demonstrated,however, that glass also has disadvantages, e.g., a naturalfluorescence. The treatment methods described hereinabove are eithercomplex, however, such as plasma treatment, or they result in anunsatisfactorily activated surface.

SUMMARY OF THE INVENTION

The goal of the present invention, therefore, is to provide glass, thesurface of which is suitable for modification and is therefore capableof being used as a substrate basis and/or carrier for a large number ofapplications in which the glass surface must be treated and/or coatedwith an agent. The glass and/or the surface should be resistant toaging.

Another goal of the present invention is to provide a method with whichglass of this nature is capable of being obtained in this manner in ahighly reproducible fashion. Finally, the goal of the present inventionis to produce glass of this nature that exhibits only minimalfluorescence when used in the typical optical techniques.

This goal is achieved with the method according to the invention asdefined in the claims. Preferred embodiments are defined in thesubclaims.

A surprising discovery was that the goal on which the invention is basedmay be achieved by adding water to melted borosilicate glass. The waterof crystallization of the starting materials, in particular, is apreferred water source. According to the invention, boric acid is usedparticularly preferably as the source for boroxide.

The method according to the invention is carried out preferably in thepresence of aqueous water in a strongly hydrous atmosphere. The melt istypically brought in contact with the hydrous atmosphere. The hydrousatmosphere used in the method according to the invention is capable ofbeing produced in various ways. The preferred procedure is to heat theglass melt in the chamber with fossil fuels, whereby combustion isinduced using pure oxygen instead of air (the “oxyfuel technique”). Inprinciple, it is also possible according to the invention, however, toheat the melt using other technical means and to introduce gaseous waterinto the atmosphere.

A further possibility for increasing the water content in the melt isdescribed in publication DE-A 100 43 454, for example. According to saidpublication, the oxygen that is released during refinement can betransferred on platinum pipes—which have been rinsed with hydrogen orwater vapor—to water that is dissolved in the glass melt.

In a further embodiment that is preferred according to the invention, aglass body is melted only on the surface, in a hydrous atmosphere. Asufficient amount of water also dissolves in the melted glass surface.It was a surprise to discover, according to the invention, that awell-modifiable surface may also be produced easily and conveniently inthis manner.

With the method according to the invention, it is possible to provideborosilicate glass that contains at least 30 mMol/liter dissolved watermolecules. At least one portion of the water is thereby chemically boundin the SiO₂ network, and reactive SiOH groups are formed.

Typically, however, glass that is treated in this manner contains atleast 35 mMol/liter, and at least 40 mMol/liter water is particularlypreferred in most cases. This means that the quantity of OH groups thatis present is at least 60, usually at least 70 and, mostly, at least 80mMol/liter. In the method according to the invention it is alsopossible, however, to easily create a higher concentration of reactiveOH groups by increasing the water-gas atmosphere or, using the methodsdescribed previously, by reacting the oxygen that is present in the melton platinum pipes rinsed with hydrogen to form water and/or OH groups.Advantageous upper limits of the water content are 75 mMol/liter, inparticular 70 mMol/liter, and 65 mMol/liter and/or 60 mMol/liter areparticularly preferred. The range of 40 to 60 mMol/liter is mostparticularly preferred.

In this manner, it is immediately possible to provide glasses that areavailable, as a substrate, for modification, and that usually have atleast 1,500 reactive spots/cm² surface. The glasses produced accordingto the invention typically contain so many reactive OH groups capable ofbinding that at least 3,000, usually at least 4,000, and regularly 5,000coating and/or modification molecules per square centimeter can be boundon the glass surface. Coating densities with spots of 7,000 to 8,000,even up to 10,000 molecules per square centimeter are possible with theglasses according to the invention. Coatings having thicknesses such asthese may be applied easily using printing techniques that are known perse. Ink-jet printers are a preferred technique.

According to the invention, it is preferable to produce glasses of thistype as float glass. In this manner, glasses can be provided that have ahighly reactive surface and a roughness of greater than 10 nm, andusually greater than 20 nm. Glasses of this type usually have a surfaceroughness of 150 nm. The high reactivity of glasses according to theinvention is that much more surprising because—as described inpublication WO 99/40038, for example—it had been assumed that glasses ofthis type must have a surface roughness of less than 10 nm.

With the method according to the invention it is possible to achieve anincrease in surface reactivity with borosilicate glasses. If hydrogen isalso added to the method, as in the case of float glass, to preventoxidation of the tin surface, then the oxygen content that is present inthe float bath atmosphere is also reduced to water. As a result, theconcentration of water gas that penetrates the melted glass and/or thefloated glass is increased further. A preferred float atmospherecontains an inert gas, such as N₂, He, etc., and a gaseous reductant,such as H₂.

Inert gas is present in an amount of 80 to 85 percent by volume, and thereduction gas is present in an amount of 5 to 15 percent by volume.

Borosilicate glass is a preferred glass according to the invention.Preferred borosilicate glass has a composition of 70 to 85 and/or up to87 percent by weight of SiO₂, 7 to 15 percent by weight of B₂O₃, 1.5 to7 percent by weight of Al₂O₃, 2 to 6 percent by weight of Na₂O, and 0 to3 percent by weight of K₂O. The amount of SiO₂ is typically 78 to 83percent by weight, and 79 to 82 percent by weight is preferred. Thecontent of B₂O₃ is typically 10 to 14 and preferably 11 to 13.3 percentby weight. The content of Al₂O₃ is typically between 1.5 and 3 percentby weight, and preferably between 1.8 and 2.6 percent by weight. Thecontent of Na₂O is typically 2.8 to 5 percent by weight, and 3 to 4.5percent by weight is preferred. Finally, the content of K₂O in theborosilicate glass according to the invention is 0 to 1.5 percent byweight, and 0 to 1.2 percent by weight is particularly preferred. In aparticularly preferred embodiment, the glass contains 80 to 81.5 percentby weight of SiO₂, 12 to 12.9 percent by weight of B₂O₃, 2.1 to 2.4percent by weight of Al₂O₃, and 3.2 to 4.1 percent by weight of Na₂O and0 to 0.95 percent by weight of K₂O. For the borosilicate glass that ispreferred according to the invention it has proven advantageous when thesum of Na₂O and K₂O is at least 2.5 percent by weight and a maximum of 8percent by weight. The minimum amount of both alkali oxides is typicallyat least 3 percent by weight and a maximum of 5 percent by weight,however, whereby at least 3.8 and a maximum of 4.5 percent by weight ispreferred. Particularly preferably, the sum of alkali oxides is at least4 percent by weight and a maximum of 4.25 percent by weight. In glassesof this nature, SiO₂ is the network former.

The method according to the invention can also be carried out withborosilicate glasses that contain alkaline-earth oxides. However, it ispreferably carried out on glasses that contain no alkaline-earth oxidesor only slight amounts thereof, i.e., impurities only.

Preferably, the borosilicate glasses used in the method according to theinvention contain no toxic refining oxides of the fifth main group withpolyvalent ionic character, such as As₂O₃ or Sb₂O₃.

In addition, it has been demonstrated that borosilicate glasses thatcontain no iron have particularly good transmission properties; this isnecessary for a large number of applications that use glass substrates,such as microarrays and biochips.

It was also found that, by selecting the appropriate raw materials forproducing the glass, the concentration of iron, in particular Fe³⁺ ions,such as Fe₂O₃, may be easily reduced to <0.015 percent by weight (150ppm). This enables glass to be obtained that exhibits extremely lownatural fluorescence. Glasses are obtainable in this manner that havehigh transparency in the ultraviolet range, particularly in the UVB/UVAwavelength. For example, transmission values of >90% at a wavelength of360 nm are achieved with floated standard thicknesses of only 0.7 mm to7 mm. Even at wavelengths of 300 nm, transmission values of 70% canstill be achieved with thicknesses of 0.7 to 1 mm.

In a particularly preferred embodiment, the glass according to theinvention contains a concentration of octahedrally bound Fe³⁺ ions of<10 ppm, and a concentration of Cr³⁺ of <10 ppm, preferably <5 ppm and,particularly, <2 ppm. The borosilicate glass obtained according to theinvention also exhibits an extremely low natural fluorescence andtherefore enables improved detection and error-free evaluation ofsignals that are emitted from fluorescent dyes, which are typically usedtoday as fluorescent markers. The working range of instruments and/ormarkers of this nature is the wavelength range of 488 nm to 633 nm. Theglass is low-fluorescence, i.e., it has such a low natural fluorescencethat, in the typical working range, it emits no noticeable naturalfluorescence or no natural fluorescence that interferes with the test.

According to the invention, it has also been demonstrated that theprocedure according to the invention eliminates the need to providesubstrate glasses that are free of alkali ions, as described inpublication WO 99/40038, for example. Surprisingly, it has also beendemonstrated that, with glasses that are produced using the methodaccording to the invention, the diffusion of sodium ions into thefunctional layers, which is described there, does not take place.

The glasses produced according to the invention exhibit a high chemicalresistance to acids and lyes, in particular alkaline lyes. They havelong-term stability as well, which also allows them to be coated easilywith a high dot and/or spot density after long-term storage.

Using the method according to the invention, in particular forproduction using the float method, a substrate glass is obtained thathas very flat, fire-polished surfaces with a mean waviness in the rangeof 0.08 μm on the underside, and 0.11 μm on the top side. Non-porous,smooth surfaces of this nature prove particularly advantageous forhybridization procedures.

The method according to the invention is capable of being used on allcommon borosilicate glasses. Their production is known to one skilled inthe art, and it can be obtained, for example, by melting a batch ofquartz sand (SiO₂), hydrated sodium tetraborate (Na₂B₄O₇), potassiumnitrate, aluminum hydroxide and common salt as the refining agent. Atypical mixture contains 70 to 87 percent by weight of SiO₂, 7 to 15percent by weight of B₂O₃, O to 8, and particularly 1 to 8, percent byweight of Al₂O₃, 1 to 8 percent by weight of Na₂O, 0.1 to 8 percent byweight of K₂O, and, if necessary, 0.1 to 8 percent by weight of othercomponents. If necessary, the SiO₂ content can also be reduced to 62 or64 percent by weight, as long as the SiO₂ still functions as a networkformer. In individual cases, up to 6 percent by weight of SnO₂, up to 4percent by weight of TiO₂, and slight amounts, i.e., up to 0.1 percentby weight, of Sb₂O₃ can be present.

Glass of this type that is obtained according to the invention can becoated in a manner known per se. According to the typical procedure, theglass surface is first cleaned. Any salts on the surface are removed.Various cleaning methods can be used here. The cleaning procedureincludes treatment with alkaline, acidic and/or organic media. A type ofcleaning that is used often is the “Kern cleaning method”, in whichrinsing is first carried out using alkaline and oxidizing solutions atelevated temperatures, followed by rinsing with water at roomtemperatures, then aftertreatment with acids is carried out at elevatedtemperatures (W. Kern and D. A. Puotinen: Cleaning solutions based onhydrogen peroxide for use in silicon semi-conductor technology, RCA Rev.(1970) 187–206). After rinsing with water, a glass surface is obtainedthat can be used to immobilize the most diverse types of reagents. Asummary of cleaning methods of this type, e.g., by J. J. Cras, C. A.Rowe-Taitt, D. A. Nievens and F. S. Ligler, is described in “Comparisonof chemical cleaning methods of glass in preparation for silanization”in Biosensors & Bioelectronics 14 (1999) 683–688. After cleaning, inparticular, the glasses obtained according to the invention are usuallycoated with a particular desired substance by covalently bondingreactive and/or functional groups of the substance to the surface viachemical reaction with the SiOH groups of the glass.

The present invention also relates to borosilicate glass and/or aborosilicate glass substrate that is obtained using the method accordingto the invention, and that has a reactive OH group density of at least30 mMol/liter, at least on its surface. It is preferably planar glass,in particular a flat glass such as float glass.

The present invention also relates to the use of glasses of this typefor the chemically covalent immobilization of reactive substances, inparticular to produce sensors and biochips, dirt-proof glasses, inparticular window glasses, such as Duran®, laboratory glasses, reagentglasses, and microarrays, such as electronic noses and/or artificialnose chips, electronic tongues, chips for the polymerase-chain reaction,DNA-microarray chips and/or gene chips, protein chips, and “biochemicallaboratories” on a chip. Chips of this nature are also used in thediagnosis and analysis of samples, and labelled samples in particular,such as fluorescence-, color-, or radioisotope-labelled samples, and ingas and smoke alarms. The glasses coated in accordance with theinvention are also suited for use in sensors in the automotive industry,such as pressure, rollover and skid sensors. The invention will bedescribed in greater detail briefly using the following examples.

EXAMPLES

The appropriate starting materials were fused together to meltborosilicate glasses that conform with the standard EN 1748-1. Theglasses were refined with common salt during melting. The borosilicateglass that was melted in this manner was then poured into a float systemto form flat glass. Thin-glass substrates with a thickness of 0.7 mm,1.1 mm, 2 mm, 3 mm and 5 mm were produced in this manner. By selectingthe appropriate raw materials, the floated borosilicate glass containedFe₂O₃ in an amount <150 ppm. The following glasses were produced in thismanner.

Composition Composition Composition Composition Composition CompositionComposition Composition Composition of of of of of of of of of Example 1Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8Example 9 SiO₂ 80.7 80.6 80.6 80.2 81.0 81.2 80.8 79.00 78.5 B₂O₃ 12.712.9 12.9 12.7 12.3 12.0 12.6 10.50 10.30 Al₂O₃ 2.4 2.25 2.25 2.10 2.32.2 2.2 4.30 4.50 Na₂O 3.5 3.2 4.05 3.60 4.0 4.1 3.8 4.70 4.50 K₂O 0.60.95 — 0.60 — — 0.4 1.00 0.70 Fe₂O₃ <0.015 <0.015 <0.015 <0.015 <0.015<0.015 <0.015 <0.015 <0.015 ρ g cm⁻³ 2.22 2.23 2.23 2.23 2.22 2.22 2.222.27 2.28 α 10⁻⁶ K⁻¹ 3.25 3.35 3.32 3.34 3.23 3.27 3.20 4.11 4.06 Tg °C. 528 530 534 515 516 541 530 563 570 OKP ° C. 560 562 556 552 556 565560 586 599 EW ° C. 825 830 815 820 832 835 820 830 838 VA ° C. 12651250 1245 1258 1266 1267 1240 1271 1286 T_(300 nm) 30.0 30.0 30.0 30.030.0 30.0 30.0 30.0 30.0

The glasses obtained in this manner were than investigated with regardfor their transmission properties. It was shown that the glassesobtained in this manner have high transparency in the UVB and UVAwavelength range. At a wavelength of 360 nm, transmission values of 90%are still obtained for the standard thicknesses mentioned previously. Ata wavelength of 300 nm, the glasses mentioned hereinabove still achievea transmission value of >70% at a thickness of 1 mm. In addition, theglasses according to the invention exhibit a slight, i.e., barelynoticeable, solarization, so that radiation damage and/or errors inlight permeability caused by solarization when strong irradiation occursat the wavelengths typically used can be disregarded. In addition, theglasses according to the invention exhibit an only slight fluorescencebehavior.

Before use, the surfaces of the substrate glasses according to theinvention are cleaned of organic and anorganic contaminants. A cleaningprocedure of this nature is preferred for the derivatization of thesurfaces. In this context, it has been demonstrated that the “Kerncleaning method”, which is known from semiconductor technology, isparticularly suited for the glasses according to the invention (W. Kernand D. A. Puotinen: Cleaning solutions based on hydrogen peroxide foruse in silicon semi-conductor technology, RCA Rev. (1970) 187–206).

1. A method of producing borosilicate glass with an easily modifiedsurface, said easily modified surface having reactive SiOH groupsthereon, said method comprising the steps of; a) making a glass meltcomprising a borosilicate glass; and b) dissolving at least 30 mMol ofwater in said glass melt per liter of said glass melt; wherein saidborosilicate glass comprises from 70 to 87 percent by weight of SiO₂,from 7 to 15 percent by weight of B₂O₃, from 0 to 8 percent by weight ofAl₂O₃, from 0 to 8 percent by weight of Na₂O, from 0 to 8 percent byweight of K₂O and optionally from 0 to 8 percent by weight of additionaloptional components.
 2. The method as defined in claim 1, wherein saidwater is added to said glass melt via hydrous starting materials.
 3. Themethod as defined in claim 1, wherein said glass melt is brought intocontact with a hydrous atmosphere, in order to dissolve said water insaid glass melt.
 4. The method as defined in claim 3, wherein saidhydrous atmosphere is produced by burning fossil fuels with pure oxygen.5. The method as defined in claim 1, wherein said borosilicate glass isa float glass and said method comprises a float glass making process. 6.The method as defined in claim 1, wherein said borosilicate glass isfree of toxic refining agents.
 7. The method as defined in claim 1,wherein said borosilicate glass contains alkali ions.
 8. The method asdefined in claim 1, wherein said borosilicate glass contains less than150 ppm of Fe₂O₃ and less than 5 ppm of Cr⁺³.
 9. The method as definedin claim 1, further comprising reacting said reactive SiOH groups onsaid easily modified surface with reactive groups of at least onesubstance in order to at least partial coat said surface with said atleast one substance.
 10. A borosilicate glass with an easily modifiedsurface, said easily modified surface having reactive SiOH groupsthereon, wherein said borosilicate glass is made by a method comprisingthe steps of: a) making a glass melt comprising a borosilicate glass;and b) dissolving at least 30 mMol of water in said glass melt per literof said glass melt; wherein said boroslilcate glass comprises from 70 to87 percent by weight of SiO₂, from 7 to 15 percent by weight of B₂O₃,from 0 to 8 percent by weight of Al₂O₃, from 0 to 8 percent by weight ofNa₂O, from 0 to 8 percent by weight of K₂O and optionally from 0 to 8percent by weight of additional optional components.
 11. Theborosilicate glass as defined in claim 10, wherein said method comprisesadding said water to said glass melt via hydrous starting materials. 12.The borosilicate glass as defined in claim 10, wherein said methodcomprises bringing said glass melt In contact with a hydrous atmosphere,in order to dissolve said water in said glass melt.
 13. The borosilicateglass as defined in claim 12, wherein said hydrous atmosphere isproduced by burning fossil fuels with pure oxygen.
 14. A substrate forchemically covalent immobilization of reactive substances, saidsubstrate comprising a borosilicate glass with an easily modifiedsurface, said easily modified surface having reactive SiOH groupsthereon, wherein said borosilicate glass is made by a method comprisingthe steps of: a) making a glass melt comprising a borosilicate glass;and b) dissolving at least 30 mMol of water in said glass melt per literof said glass melt; wherein said borosilicate glass comprises from 70 to87 percent by weight of SiO₂, from 7 to 15 percent by weight of B₂O₃,from 0 to 8 percent by weight of Al₂O₃, from 0 to 8 percent by weight ofNa₂O, from 0 to 8 percent by weight of K₂O and optionally from 0 to 5percent by weight of additional optional components.
 15. A sensor,biochip, dirt-proof working glass for a window, reagent glass,laboratory glass, micro-array, electronic nose, artificial nose chip,electronic tongue, chip for polymerase-chain reaction, DNA micro-arraychip, gene chip, protein chip or a biochemical laboratory chipcomprising said substrate as defined in claim 14.